1. Field of the Invention
The present invention relates generally to an apparatus and method for heating a workpiece and transferring the heated workpiece to a forming press; and, more specifically, to an apparatus and method to preheat a workpiece and transfer the preheated workpiece to a forming die wherein the workpiece undergoes a superplastic forming process.
2. Description of Related Art
Superplastic forming (SPF) takes advantage of a material's superplasticity or ability to be strained past its rupture point under certain elevated temperature conditions. Superplasticity in metals is defined by very high tensile elongations, ranging from two hundred to several thousand percent. Superplasticity is the ability of certain materials to undergo extreme elongation at the proper temperature and strain rate. SPF is a process used to produce parts that are difficult to form using conventional fabrication techniques.
SPF typically includes the steps of heating a sheet of material to a point of superplasticity, clamping the material within a sealed die and then using gas pressure applied to one side of the sheet of material to force the material to stretch and take the shape of a forming surface located in the die cavity. At higher temperatures, superplastic materials may stretch several times their initial length without breaking. Controlling the gas pressure during the forming process controls the deformation rate of the material and maintains superplasticity at the elevated temperature.
Typical SPF applications, while having advantages over conventional stamping techniques including increased forming strains, reduced spring back and low tooling costs, have disadvantages in that they are limited to low volumes as they have relatively long forming cycle times. Specifically, a conventional SPF process used to manufacture a complex part can require a forming cycle time as high as 30 minutes.
Reduced cycle times are necessary in order to use SPF for the high production requirements of the automotive industry. Prior art SPF forming processes typically start with loading a room temperature metal sheet or blank into a heated forming die located in a press assembly used to open and close the forming die. The heated forming die operates to heat the metal sheet, typically by a combination of conduction and convection, to a forming temperature. This step automatically builds a certain amount of the dwell time into the process before the forming cycle begins. Accordingly, using the forming die to heat the metal sheet further increases the overall cycle time used to manufacture a part. Further, heating the metal sheet with the forming die is not as efficient as other heating methods.
An apparatus and method for loading a preheated workpiece into a forming die of a superplastic forming apparatus can significantly reduce overall cycle times by using the time the workpiece spends in the forming die for forming, not waiting for the workpiece to reach suitable SPF forming temperatures. Accordingly, such an apparatus and method is advantageous in that it helps to increase the production volumes obtained using a superplastic forming manufacturing process.